1. Field of the Invention
The invention relates in general to a method of improving junction leakage problem of shallow trench isolation (STI) structure, and more particularly to a method of improving junction leakage problem as STI and self-aligned silicide (Salicide) processes are performed.
2. Description of the Related Art
Integrated circuits (IC) are composed of many devices and isolation structures formed between the devices. The isolation structures, such as STI structure or field oxide isolation structure, are used for preventing carriers moving between devices and reducing charge leakage problem. As IC devices are scaled down, STI is currently employed in manufacturing IC devices, particularly in sub-half micron semiconductor process, to avoid the topographical uncertanties caused in using the more conventional thick field oxide isolation. The conventional growth of thermal field oxide using a mask such as nitride creates an encroachment of the oxide into the active areas, this encroachment being referred to as the bird's beak effect.
When STI is used, it is common to use wet dip (or wet etching) for removing oxide. Trench sidewall of STI structure may accordingly exposed after wet oxide dip (or wet oxide etching) to result in recess effect on the upper surface of the trench sidewall, such as sidewall recess 16 as shown in FIG. 1. It is more serious for non-volatile products, such as flash memories, since the non-volatile products include many oxide layers. For example, there are at least tunnel oxide layer, silicon oxide/silicon nitride/silicon oxide (ONO) layer or several different gate oxide layers with different thickness formed in flash memories. Therefore, the wet oxide dip process will perform much more times during fabricating non-volatile products.
As IC devices are scaled down, STI and Salicide processes are usually combined to reduce the resistance of IC devices. Salicide process is used for reducing the resistance of conductive poly-silicon layer and wiring lines. The Salicide process comprises the steps of forming refractory metal layer on poly-silicon layer or silicon substrate, and performing thermal process to make the metal layer reacting with silicon to form metal silicide (salicide) on the poly-silicon layer or silicon substrate. Once Salicide process performing with STI process, salicide layer will form on the sidewall recess in the trench that is not enclosed by the junction. Such salicide layer will cause serious leakage problem.